1. Field of the Invention
The present invention relates to a roller screw for linear transmission, and more particularly to a roller screw having plural circulating devices cooperating with plural helical grooves.
2. Description of the Prior Art
As for the design of controlling the movement of the stages or the processing elements in the precision machines, in order to control the feed amount precisely and reduce the friction coefficient, a roller screw is commonly used for transmission. Though the circulation systems are various in structures, they are generally categorized into two types: one type is one screw cooperating with one nut, and the other type is one nut cooperating with two nuts. The structure design of providing helical grooves cooperating with plural rollers between the screw and the nut generally has the following types:
A. One screw cooperates with one nut, each of the screw and the nut is provided with a helical groove, and plural rollers are arranged in parallel in the helical grooves. By using such a single helical groove, the rolling direction and angle of all the rollers are the same, so that when the nut rotates, the screw is unilaterally subjected to a force along the moving direction of the rollers in the helical grooves, it thus is only able to bear a unidirectional load. As a result, this design can only be applied to a machine which is unilaterally subjected to a force (such as: injection molding machine, mold closing machine, etc)
B. One screw cooperates with a nut, each of the screw and the nut is provided with one helical groove, and plural rollers are arranged in the helical grooves in a stagger manner. Though this stagger arrangement can achieve the objective of sharing the load evenly in two directions, the number of the rollers bearing the load in each direction is only half of the total rollers (because the rollers are arranged in the stagger manner), so this design still has poor loading capability, namely, poor rigidity.
C. One screw cooperates with two nuts, each of the screw and the nuts is provided with one helical groove, and plural rollers are arranged in the helical grooves parallel to one another. In such a structure design of a single nut cooperating with two nuts, the rollers with different tilting angles are respectively arranged in two nuts, so that the roller screw can bear the load in two directions. However, since there are two nuts, the total length of the nuts on the screw is relatively increased, thus relatively increasing the manufacturing and material cost. In addition, the contact surface between the two nuts requires much higher surface precision and parallelism, so the fabrication difficulty is increased, and the installation of the nuts is very complex and time-consuming, thus increasing the fabrication cost.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.